We don't have 4-way-stops in Germany, too, but there is the possibility, that 4 cars are meeting at the crossing of two smaller street, where there aren't no signs and the "righ first"-rule goes. Then you have to act like at a 4-way-stop.

I use my horn rarely. It sucks, if people are using them to greet some friends, they are seeing on the street.
The last time I've used the horn last week in our village. In front of the supermarket, where the boardwalk was fully parked, an older lady was suddenly going on the road, pushing an overloaded bicycle. She act in such a stupid way, that the cyclist in front of me nearly was falling on the street. She don't took a lok around, just entering the narrow street. I used the horn just 2 meters behind her. She was so alarmed, that the nearly had her bicyle let falling on the street.